1. Field of the Invention
The present invention relates to a method of low-temperature and neutral-pH precooking for the production of corn flour and, more particularly, to one that achieves continuous partial hydrolysis of the insoluble heteroxylans, starchy and proteinaceus bran cell-walls while avoiding excessive pregelatinization with a xylanase, endoamylase and endoprotease blend as a processing aid during the manufacture of an instant corn flour for the elaboration of snack and tortilla foods.
2. Description of Related Art
The production of high-quality masa flour can be produced by conventional techniques only if the food-grade dent corn has the following characteristics: uniformity in kernel size and hardness, low number of stress-cracks and kernel damage and ease of pericarp removal during the lime-water cooking process. The mature kernel has four separable components, on a dry weight basis: tip cap (0.8-1.1%), pericarp or bran (5.1-5.7%), endosperm (81.1-83.5%), and germ (10.2-11.9%). In dry or wet-milling processes the bran includes the pericarp, tip cap, aleurone layer (isolated with bran) and adhering pieces of aleurone/starchy endosperm as well. A native corn bran contained starch (4-22%) and protein (5-8%) arising from the endosperm tissue and glycoprotein pericarp as well (Saulnier et al. 1995 and Hromadkova et al. 1995). Nixtamalized corn flour (NCF) is produced by the steps of alkaline cooking of corn, washing, milling the nixtamal and drying to give corn masa flour. This flour is sieved and blended for different product applications and it is usually supplemented with additives before packaging for commercial table or packaged-tortilla and snack production. Although the pericarp or bran is partially removed during the alkaline-cooking and washing process stages, there is still fiber left from the corn kernel (Montemayor and Rubio, 1983, U.S. Pat. No. 4,513,018). Whole Nixtamalized corn flour or masa flour can contain from 7-9% of total dietary fiber or bran with 6-8% mainly consisting of insoluble fiber on a dry basis (Sustain, 1997, U.S. Pat. No. 6,764,699).
The cell walls or non-starch polysaccharides (NSP) are the major corn dietary fiber components and are composed of hemicellulose (heteroxylan or pentosan and β-glucan: 4.4-6.2%), cellulose (2.5-3.3%) and some lignin (0.2%). According to Watson (1987: Tables Iv and VII), the corn pericarp/tip cap makes up about 5-6% and aleurone-endosperm has about 2% of the kernel dry weight. This pericarp also contains 90% insoluble fiber (67% hemicellulose and 23% cellulose) and only 0.6% soluble-fiber (soluble-arabinoxylan and β-glucan). It is estimated that dietary fiber in both pericarp or bran (4.9%) and endosperm (2.6%) make up 80% of the total dietary fiber. The corn insoluble fiber is mainly found in the pericarp and endosperm (aleurone and starchy) which make up 68% of the total dietary fiber (9.5% in a dry-weight basis). The corn bran layers comprise the outer (beeswing or hull), inner (cross and tube cells), nucellar layer and endosperm (aleurone and starchy) cell-walls. The innermost tube-cell layer is a row of longitudinal tubes pressed tightly against the aleurone layer. Next there is a very loose and open area called the cross-cell layer, which has a great deal of intercellular space. These areas provide capillary interconnections between all cells, which facilitate water absorption. The pericarp extends to the base of the kernel, uniting with the tip cap. Inside the tip cap there are spongy-branched cells openly connected with the cross-cells.
Unlike corn endosperm, in which soluble fiber amounts to 12% of the total fiber (4.1%), in whole wheat, soluble fiber represents 22% of total fiber (about 20% of the flour water-uptake is bound to the soluble pentosan fraction). Arabinoxylan is a complex polymer (20,000-170,000 Daltons) with a linear backbone of (1,4)-β-xylopiranosyl units to which substituents are attached through O2 and O3 atoms of the xylosil residues (mainly, α-L-arabinofuranosyl). This polymer is apparently linked to the cellulose skeleton in the corn cell wall by ester linkage cross-bonding through ferulic and diferulic acid (Watson, 1987). However, heteroxylan insolubility in corn bran might be due to protein-polysaccharide linkages (pericarp glycoproytein or extensin) and a highly branched structure (23% of the xylan backbone does not bear side-chains) as opposed to wheat bran (Saulnier et al., 1995).
During alkali-cooking and/or steeping, there are chemical and physical changes such as nutrient losses along with partial pericarp or bran removal, degradation of the endosperm periphery with starch gelatinization/swelling and protein denaturation in the precooked corn kernel. The most important nutritional modifications are: an increase in the calcium level with improvement in the Ca to P ratio, a decrease in insoluble dietary fiber and zein-protein, a reduction in thiamin and riboflavin, an improvement of the leucine to isoleucine ratio reducing the requirement for niacin, and leaching the aflatoxins into the wastewater (Sustain, 1997).
The known cooking methods (batch or continuous) have been proposed as the critical variables (Sahai et al., 2001) which determine soluble-solid loss (1% to 1.6% COD) in limewater residue for anaerobic biodegradation (Alvarez and Ramirez, 1995). Dry solid matter (1.5%-2.5%) includes an average of 50-60% dietary fiber, 15-20% ash, 15% starch, 5-10% protein and 5% fat. Bryant et al., (1997) showed an optimum change in starch behavior at a lime level similar to the corn masa industry where starch gelatinization indicators (enzyme digestion, water retention capacity, starch solubility and DSC-peak temperature=69° to 75° C.) are increased with lime addition of 0% to 0.4%, peaking at 0.2%. They also found a peak-viscosity temperature reduction upon the addition of lime up to 0.5%, indicating faster granule swelling that requires less thermal energy. Corn pericarp nixtamalization (Martinez et al., 2001) has a first-order stage associated with a fast dissolution of hot-water soluble fractions as starch and pectin, and alkali-soluble fat. A second stage is due to a slow alkaline-hydrolysis of the hemicellulose-cellulose-lignin structure with a higher hemicellulose loss proportional to alkaline-pH concentrations.
Arabinoxylan degrading enzymes include xylanases (1,4-β-D-xylan xylanohydrolase, EC 3.2.1.8) and β-xylosidases (1,4-β-D-xylan xylohydrolase, EC 3.2.1.37). The former endoenzyme randomly hydrolyze the insoluble and soluble xylan backbone (EC 3.2.1.8) whereas the latter exoenzyme hydrolyze xylose from the non-reducing end of the xylose-polymer (EC 3.2.1.37). Xylose is not usually the major product and it is typically produced after xylobiose and xylotriose (smallest oligomer). Virtually all xylanases are endo-acting as determined by chromatography or their kinetic properties (substrate and product formation), molecular weight and pH (basic or acidic) or its DNA sequence (crystal structure). They can be structurally classified into two major families or isoenzymes (F or 10 and G or 11) of glycosyl hydrolases (Jeffries, 1996). F10 xylanases are larger, with some cellulase activity and produce low DP oligosaccharides (less specific); F11 are more specific for xylan and with lower molecular weight (i.e., B. Circulans and T. harzianum).
In addition, the Enzyme Technical Association (ETA, 1999; FDA, 1998) classified as carbohydrases the following hemicellulases (trivial name): a) endoenzymes (EC 3.2.1.32=1,3-β-xylanohydrolase, 78=mannanohydrolase and 99=arabinohydrolase) and b) exoenzymes only attack branches on the xylose-polymer (pentosan), producing xylo-oligomers (EC 3.2.1.55=a-L-arabinofuranosidase) or producing acids (glucuronic-acid glycosilase and ferulic-acid esterase). Currently recognized endoenzymes (xylanases) and exoenzymes produced from A. niger (EC 3.2.1.8 and 37,55), A. oryzae (EC 3.2.1.8 or 32), B. subtilis (EC 3.2.1.99), and Trichoderma longibrachiatum (formerly reseei: EC 3.2.1.8) are Generally Recognized As Safe (GRAS; 21 CFR 182, 184 and 186) and require no further approval from the U.S. Food and Drug Administration or Recognized As Safe (RAS in Europe: Mathewson, 1998). However, direct and indirect food additives (i.e., packaging materials) are regulated in 21 CFR 172 and 174-178 as well. Secondary direct additives, a sub-class of direct additives, are primarily Processing Aids which are used to accomplish a technical effect during food processing but are not intended to serve a technical or as a functional additive in the finished food. They are also regulated in 21 CFR 173 (Partial List of Enzyme Preparations that are used in foods). Finally, all GRAS Substances produced through recombinant-DNA which were widely consumed prior to 1958, and which have been modified and commercially introduced after 1958 must comply with regulatory requirements proposed in 21 CFR 170.3 (GRAS Notice).
The benefits of using a commercial xylanase (endoenzyme) in cereal flours instead of a non-specific hemicellulase (exo- and endoenzyme) preparation are a reduction in side activities (cellulase, beta-glucanase, protease and amylase) and a reduction of dough-stickininess. Arabinoxylan degrading enzymes with well defined endo-acting and exo-acting activities have become commercially available, for food and feed, from the following companies: Amano, Danisco-Cultor, EDC/EB, Genencor, Gist-Brocades-DSM, logen, Novozymes, Primalco, Quest, Rhodia and Rohm. Suggested applications for commercial xylanases (endopentosanases) and hemicellulases (pentosanases) mentioned in the literature include: 1) improving the watering of spent grains and energy reduction during grain drying; 2) facilitation of dough formulation with less water, reduction of stickiness in noodle and pasta production; 3) reduction in the water content when formulating grains for flaking, puffing or extrusion; 4) retarding staling or hardening in bread; 5) relaxing dough for cookie and cracker production and use of sticky cereal flours in new product formulations; 6) increase in bran removal when added to tempering water; and 7) reducing both steeping time and starchy fiber in corn wet milling.
A complex set of conditions determines bakery product shelf life, so the food formulator has three basic approaches to crumb softness: prevent moisture transfer; prevent starch recrystallization; and hydrolyze starch. Crumb staling is marked by many physicochemical changes which occur in the following order: hardening and toughening of the crumb (starch retrogradation); appearance of crumbliness; and moisture loss by evaporation. Commercial amylases act as anti-staling agents by breaking down gelatinized starch during baking. Some commercial microbial amylases (ETA, 1999;FDA, 1998) are listed by name and source are: a) endo-amylase (A. oryzae/niger, and R. oryzae/niveus: EC 3.2.1.1); b) exo-glucoamylase/exo-amyloglucosidase (R. oryzaelniveus and Aspergillus oryzae/niger: EC 3.2.1.3); and c) endo-pullulanase and endo-amylase (B. subtilis, B. megaterium, B. stearothermophilus and Bacillus spp.: EC 3.2.1.33,41/60 and EC 3.2.1,133). Genetic engineering technology has been used to develop amylases with endo or exo-acting (maltogenic) activity with intermediate thermostability (<65° C.) and B. stearothermophilus falls into this category. These novel amylases are fully inactivated during baking while yielding a soft crumb without gumminess even at higher dosages. Lopez-Mungia et al. (MX Patent 952,200) described an enzymatic process (with endoamylases) to obtain corn tortillas (from nixtamal or nixtamalized corn flour), which delays staling during frozen storage. Furthermore, the commercial enzyme products normally contain one or more enzymatic activities such as: carbohydrase (amylase, xylanase), protease and esterase. These hydrolases are enzymes catalyzing the hydrolysis of various bonds: EC 3.2 Glycosylase is a carbohydrase acting on O-glycosyl bonds (EC 3.2.1) and EC 3.4 Peptidase is a protease acting on peptide bonds. The proteases are further divided into “exopeptidases” acting only near a terminus of a polypeptide chain (aminoacid polymer/protein) and “endopeptidases” acting internally in polypeptide chains. The usage of “peptidase” is synonymous with “protease” as it was originally used. Two sets of sub-classes are recognized, those of the endoproteases (EC 3.4.21-24/99) and those of the exoproteases (EC 3.4.11-19). The endoproteases are mainly divided on the basis of catalytic mechanism and specificity: Serine proteases (EC 3.4.21 acting at alkaline-pH), Cysteine/Metallo proteases (EC 3.4.22/24 acting at neutral-pH) and Aspartic proteases (EC 3.4.23 acting at acid-pH). Microbial proteases fall into three broad groups: a) acid proteases with maximal activity between pH 2 and 4, b) neutral proteases at pH 7-8, inhibited by metal-chelating agents and c) alkaline proteases at pH 9-11, cleaving a wide range of peptide and ester bonds as well. Bacillus endoproteases can be divided in two types: A-group (B. subtilis, B. licheniformis/B. pumilus: <50° C.) does not produce neutral protease or amylase, and B-group (B. subtilis NRRLB3411, B. amyloliquefaciens and B. subtilis var. amylosacchariticus) produces a neutral (B. megaterium/stearothermophilus: ˜70° C.) and endo/exoamylase as well (Keay et al., 1970a,b).
A moderate hemicellulase addition decreases water uptake in wheat dough, whereas using a xylanase increases water binding and soluble-xylans as well for a high-moisture bread product. On the contrary, if starch gelatinization is to be minimized, a higher endoenzyme or xylanase addition is desirable and hydrolysis of the soluble fraction releases water for low-moisture cookie or cracker products (EPA Patent 0/338787). Therefore, a suitable level of xylanase results in desirable dough softening without causing stickiness, thereby improving machinability during forming and baking operations. Haarasilta et al. (U.S. Pat. No. 4,990,343) and Tanaka et al. (U.S. Pat. No. 5,698,245) have proposed that a preparation of hemicellulase or pentosanase with a cellulase (Cultor/Amano) causes decomposition of wheat insoluble fiber for bread volume increase. Rubio et al. (U.S. Pat. No. 6,764,699) have improved the flexibility and elasticity of packaged corn tortillas after 7 days of ambient storage by adding a fungal mix of xylanase and cellulase (>100 ppm) to a whole nixtamalized corn flour.
Antrim et al. (CA Patent 2,015,149) disclosed a process of preparing a shredded, farinaceous product by cooking whole grain (wheat), treating it with a microbial isoamylase, tempering (i.e., holding) and forming in order to bake or toast the shredded wheat product. Tobey et al. (U.S. Pat. No. 5,662,901) have used an enzyme formula (>200 ppm) and conditioned the wet or soaked grain (sorghum) for at least 30 minutes. The microbial enzymes comprised a hemicellulase, an amylase, a pectinase, a cellulase and a protease to increase both animal weight gain and feed use efficiency. Van Der Wouw et al. (U.S. Pat. No. 6,066,356) also reported the use of a recombinant-DNA endo-arabinoxylanase (Gist Brocades) breaks down the water-insoluble-solids (˜1.5%) from degermed maize and increases their in-vitro digestion (13%-19%) for animal feed or in wheat flour for improving its bread volume (˜9%).
A pilot process (WO Patent 00/45647) for the preparation of a modified masa foodstuff used a reducing agent (metabisulfite) or an enzyme as a processing aid (disulfide isomerase or thiol-protease/Danisco) with masa or corn prior to nixtamalization such that the native protein is modified. Jackson et al. (U.S. Pat. No. 6,428,828) disclosed a similar process where whole-kernel corn was steeped and digested with a food-grade commercial alkaline-protease (<1000 ppm: 50° C.-60° C.; pH>9) which altered zein structure similarly to alkali-cooking with a partial gelatinization (˜20%-40%).
A novel transgenic thermostable-reductase enzyme was cloned in corn (high-protein) mainly to enhance extractability and recovery of starch and protein important in flaking grit production and in masa production. Reduction of protein disulphide bonds alters the nature of corn flour (as a wheat substitute from high-protein corn) when steeping the corn grain between 45° C. and 95° C. instead of using sulfite salts. The critical steeping is required to soften the kernel and then to loosen starch granules from the complicated matrix of proteins and cell wall material that makes up the corn endosperm (WO. Patent 01/98509).
Tortilla is the main edible corn product in North and Central America. It is a flat, round, unleavened and baked thin pancake (flat-cornbread) made from fresh masa (ground nixtamal) or corn dough prepared from nixtamalized corn flour (masa flour). It might be mentioned that tortilla, when manually or mechanically elaborated and without additives of any kind, has a maximum shelf life of 12 to 15 hours at room temperature. Afterwards they are fermented or spoiled by microorganisms and becoming hard/stale (starch-protein aggregation) due to a physicochemical change in the starch constituent of either stored or reheated tortilla. It is known that tortillas when kept under conditions in which no moisture is lost (plastic package), nevertheless become inflexible with time and break or crumble easily when bent. In northern South America, particularly in Colombia and Venezuela, hard endosperm corn is processed with dry milling technology without wastewater and it is further converted into a precooked, degermed and debranned flour for traditional foods. Its consumption is mainly in the form of “arepa”, which is a flat or ovoid-shaped, unleavened, and baked thick-pancake made from instant flour. In other South American countries, corn meal (polenta) and corn flour as well are used for empanada, pancake and snack food (FAO, 1993).
Food fermentation processes are reliant on both endogenous and microbial enzymatic activities for the degradation of fibers, starches, proteins, anti-nutritional and toxic factors. In some cases, microbial processes are associated with indigenous fermentation processes, which exhibit unique properties. Microorganisms are currently the primary source of industrial enzymes: 50% are derived from fungi and yeast; 35% from bacteria, while the remaining 15% are either of plant or animal origin. Microbial enzymes are commercially produced either through submerged fermentation or solid-substrate fermentation technologies. The use of biocatalysts or enzymes has the potential to increase productivity,-efficiency and quality output in agro-industrial processing operations in many emerging countries. These biochemical processes generally have requirements for a simple manufacturing base, low capital investment and lower energy consumption than other food processing unit operations. Alkaline and neutral-pH fermentations of various beans (soy and locust), seeds, and leaves provide protein/lipid rich, flavorsome, low-cost food condiments to millions of people mainly in Africa and Asia (Nigerian dawadawa/ogiri, Sierra Leone ogiri-saro, Japanese natto, Indian kenima, Indonesian cabuk/semayi). Based upon the use of Bacillus spp. (B. subtilis B. licheniformis, B. pumilus), the fermentations are primarily proteolytic, yielding amino acid/peptide-rich mixtures without microbial amylase and lipase activities mainly in seeds (Steinkraus, 1996). Pozol is a fermented corn doughball (from nixtamal or lime-treated maize) produced and consumed, as a beverage/porridge, by the indigenous and mestizo population in S.E. Mexico. It is a probiotic fermentation involving at least five interacting groups which include the natural flora from a freshly prepared dough or nixtamalized corn flour (heat-resistant Bacillus spp. and Actinomycetes spp.). Agrobacterium azotophilum (reclassified as Bacillus subtilis: NRRL B21974) and K pneumonia (E. aerogenes), both of which grow in nitrogen-free media and increase the aminoacid nitrogen and likely the total-nitrogen during this solid-substrate fermentation. The other groups include a lactic-acid bacterium (amylolytic Lactobacillus sp.), which increases its flavor (0.7% lactic-acid) while lowering the alkaline pH (from 8 to ˜5 at 24-48 hours); C. tropicalis which contributes to an alcoholic/fruity aroma, and G. candidum which produces aroma and spongy texture (Ramirez and Steinkraus, 1986; Steinkraus, 2004). On the other hand, a corn wet-milling process for starch production involves an acid (pH<5) fermentation during steeping or soaking whole corn kernels counter-currently (24-48 hours at 45-50° C.). The purpose is to soften the endosperm and to break the disulfide-bonds holding the protein matrix together. Steeping is a diffusion limited unit operation where two steep-water chemical and biochemical aids are required (with ˜0.10-0.25% sulfur dioxide and ˜0.5-2.0% lactic-acid usually produced by Lactobacillus spp.). They can diffuse into the corn kernel through the base end of the tip cap, move through the cross and tube cells of the pericarp to the kernel crown and into the endosperm (Watson, 1987). The main result of a lactic fermentation is a dispersion of endosperm protein/zein and an enhancement of starch release during subsequent milling for acid-fermented corn gruels/porridges such as: Ghanian kenkey, Nigerian ogi (industrial), Kenyan uji and South African mahewu (Steinkraus, 1996 and 2004).
Properly processed industrial corn or masa flour simplifies the production of tortilla and snack products, because the customer eliminates management techniques required for wastewater treatment, securing, handling and processing corn into fresh masa for tortillas and snacks. However, an instant corn flour might have the following quality and cost disadvantages: high cost, lack of flavor and poor texture in masa and third-generation (3G) corn foods. These may include extrusion cooking, followed by cooling, holding (aging) and drying to make “snack pellets” which are expanded by frying to make the final snack product. Another example is breakfast cereals made by cooking whole grain (wheat, rice, or corn), followed by cooling, tempering (conditioning), shredding, forming into “biscuits” and baking.
Corn processors can generate added value from their industrial operations in one of three approaches: developing new products from new hybrids, increasing the yield of traditional products from corn, and improving process efficiency at a lower unit cost. In the past, this has been done by methods and using an apparatus in which the grain is cooked and/or steeped in a lime-water solution such as those disclosed in U.S. Pat. Nos. 2,584,893, 2,704,257, 3,194,664, and 4,513,018. These prior art methods for the industrial production of corn dough or masa flour involve accelerated cooking and steeping times with large amounts of solids losses (˜1.5-2.5%) in the liquid waste. In addition, essential nutrients such as vitamins and some amino acids are lost, depending on the severity of the cooking, washing and drying operations.
Many and varied methods for the production of instant corn flour for food products involving reduced amounts of water with low-temperature cooking and short-time processing for a high yield of the end product have been developed, as reflected by the following U.S. Pat. Nos. 4,594,260, 5,176,931, 5,532,013, and 6,387,437. In this connection, reference is made to the following U.S. Pat. Nos. 4,594,260, 5,176,931, 5,532,013, and 6,265,013 also requiring a low-temperature drying. On the contrary, U.S. Pat. Nos. 4,513,018, 5,447,742 5,558,898, 6,068,873, 6,322,836, 6,344,228 and 6,516,710 used a high-temperature dehydration or cooking in place of a low-temperature cooking.
Having in mind the disadvantages of the prior art methods, several studies not only have used a low-temperature precooking with minimum wastewater, but also separate corn fractions as reflected by the following U.S. Pat. Nos. 4,594,260, 5,532,013, 6,025,011, 6,068,873, 6,265,013, 6,326,045 and 6,516,710.
A few applications for enzymatic steeping were also tested to convert a traditional masa processing with reduced wastewater into a novel biochemical process (WO Patent 00/45647 and U.S. Pat. No. 6,428,828). Although the above described prior art methods are capable of either an acid or an alkaline-enzymatic precooking or steeping of the whole corn for either modified masa or masa flour processing, a continuous industrial application using instead a blend of xylanase, an endoamylase and an endoprotease as a processing aid, at a neutral-pH, was still unavailable in the market at the time of the invention.